1. Field of the Invention
The invention concerns a process for burning fluid fuels such as pulverized coal in suspension in air and a turbulent-flow burner for implementing this process.
2. Description of the prior art
The term turbulent-flow burners designates burners in which a fluid fuel such as pulverized coal in suspension in a primary air flow is introduced into a combustion zone by means of a nozzle and in which the secondary air needed for burning the fuel is caused to swirl around the end of the nozzle, for example by means of deflector plates usually called swirl vanes. A burner of this kind is described in French Pat. No. 2,054,741, for example.
These burners impose on the combustion products a vortex movement (usually called "swirl") which brings about intensive internal recirculation of the fuel and the gases, improving combustion and procuring vigorous intermixing of the products. This motion is characterized by the "swirl number" which represents the ratio of the angular momentum flowrate to the axial motion quantity flowrate for a given radius of the flow of products discharged from the burner.
In certain cases the use of this type of burner makes it difficult to obtain a flame which is stable and which is not excessively cooled by radiation to the walls of the combustion zone and by recirculation of external gases into the flame, the consequence of which is reduced combustion efficiency. Moreover, the resulting flame has a relatively large diameter and it may be desirable to confine it within as small a volume as possible, especially if the burner is used in a compact combustion zone such as a dryer drum.
It has already been proposed in French Pat. application No. 2,564,950 to limit the volume of a turbulent-flow burner flame by passing it into a confinement chamber. The walls of such a chamber may, however, be raised to a temperature causing them to be fouled by the adhesion of hot ash particles and to deteriorate rapidly, despite the use of refractory materials.
An object of the present invention is to propose a combustion process and a burner implementing this process which make it possible to circumvent the above disadvantages and consequently to achieve substantially complete combustion of the fuel in a highly stable and compact flame, also avoiding deposits of solid materials on the walls of the chamber and the combustion zone.
Another object of the invention is to propose a burner which can function without supporting the fuel and without preheating of the combustion air, in other words in which the stability of the flame is independent of the thermal conditions imposed by the combustion chamber.
These objects are achieved if there is provided around the flame as aerodynamic complementary air jacket which isolates the combustion chamber and within which the fuel is virtually completely combusted.